Issue 3: Changes in the water table

Lowering the groundwater table by only a few metres can adversely affect existing users of groundwater whether it is required as drinking water for humans and animals or to sustain plant life (particularly wetlands), especially at dry times of the year. Springs fed by groundwater will finally dry up if the level falls. Similarly low flows in rivers will be reduced.

Any changing availability of groundwater for drinking water supply needs to be assessed in terms of the economics of viable alternatives. People on low incomes may be disproportionately disadvantaged. They may also be forced to use sources of water that carry health risks, such as guinea worm infection and schistosomiasis. In parts of Asia there are indications that lowering the ground level may favour sandfly which may be vectors for diseases such as visceral leishmaniasis.

Saline intrusion in coastal locations is a problem associated with lowering groundwater levels and can have severe environmental and economic consequences.

A continued reduction in the water table level (groundwater mining), apart from removing an important resource, may lead to significant land subsidence with consequent damage to structures and difficulties in operating hydraulic structures for flood defence, drainage and irrigation. Vulnerable areas are those with compressible strata, such as clays and some fine-grained sediments. Any structural change in the soil is often irreversible. The ground level can fall with a lowering of the water table if the soils are organic. Peats shrink and compact significantly on draining, with consequent lowering of the ground level by several metres. Particular care is needed in the drainage of tropical coastal swamp regions as FeSO₄ soils can become severely acidic resulting in the formation of "cat-clays".

A number of negative consequences of a falling water table are irreversible and difficult to compensate for, e.g. salt water intrusion and land subsidence.

Reference:

Dougherty, T.C., Environmental impact assessment of irrigation and drainage projects, 53 FAO Irrigation and Drainage Paper, 1995.

GIWA definition

Lowering of the water table refers to the depletion of aquifers as a direct or indirect consequence of human activity.

Environmental impacts

1. Reduction in stream flows

2. Land subsidence

3. Reduced aquifer capacity

4. Reduced vegetation cover

5. Salt water intrusion potential

6. Water quality changes

7. Increased soil erosion

8. Increased penetration of contaminants into deep aquifers

Indicators

The following are environmental impact indicators for changes in the water table.

Select the environmental impact indicators that are relevant from the list below and input data into the appropriate indicator sheets.

Hydrological

Map satellite derived

Parameters

Pollution

Biological